Western lowland gorillas, one of four gorilla subspecies, inhabit swampy forests only a few hundred feet above sea level in the Congo Basin of central Africa. They live and travel in small groups that consist of a silverback male, his female mates, and their offspring.

Although there are tens of thousands of western gorillas left in the wild, the World Conservation Union (IUCN) recently changed the status of the great apes from “endangered” to “critically endangered.” One of the major threats is the Ebola virus, which has killed up to 95 percent of the apes in areas of Gabon and the Republic of the Congo over the past 20 years.

Researchers aren’t exactly sure how gorillas acquire and pass on Ebola, but they suspect that the virus rapidly became widespread among the western lowland subspecies because of their feeding and ranging habits. The gorillas travel long distances every day to find food, especially fruit, and their paths often overlap: Individuals from different groups frequently feed at the same trees and in the same swamps. It could be that the infected gorillas leave tainted urine, saliva, and feces on and around trees that are then visited by other gorillas. Researchers have also documented that gorillas are curious about their dead and inspect carcasses they come across, putting them at risk of contracting Ebola.

Scientists are working on an Ebola vaccine for gorillas and chimpanzees. It’s still in the development stage, but several experimental vaccines tested on laboratory monkeys have proved successful. In the works are field trials to establish the best way to deliver a vaccine, either by darting or by oral baiting. These trials are in their earliest stages—no actual vaccine is used—and focused on determining how difficult it is to dart the notoriously elusive gorillas and which baits they’ll eat.